1. Field of the Invention
The present invention relates to a process for producing a pressure-sensitive copying paper. In more detail, the present invention relates to a process for producing a pressure-sensitive copying paper which comprises coating a coating solution for a pressure-sensitive copying paper onto a long support (hereafter referred to as a "web") which is continuously running.
2. Description of the Prior Art
Known types of pressure-sensitive copying papers include types comprising a support having thereon a coating layer comprising microcapsules, a binder and a protective agent on one side thereof and a coating layer containing a color developer as a main component on the other side thereof, and types comprising a support having thereon only either a coating layer comprising microcapsules, a binder and a protective agent or a coating layer containing a color developer as a main component on one side thereof. These types are utilized by contacting the coating layer containing microcapsules face-to-face with the coating layer containing a color developer.
The reason why a protective agent is incorporated is to prevent the pressure-sensitive copying paper from being stained due to the formation of an undesired coloration caused by pressure or abrasion occurring during handling, rather than the letter marking for which the paper is designed, prematurely rupturing the microcapsules and as a result a color former in the destroyed microcapsules reacts with a color developer to form a color.
In this case, an air knife coating method (for example, as described in U.S. Pat. Nos. 3,186,851 and 3,472,674, British Pat. No. 1,176,469, etc.) or a blade coating method (for example, as described in Japanese Patent Publication 35330/74, British Pat. No. 1,339,082, etc.) have hitherto been employed exclusively as a coating means. Such methods all comprise coating an excess amount of a coating solution onto a web and then scraping the excess coating solution off by means of an air knife or a blade, thereby adjusting the coating to a desired amount of the coating solution and at the same time, the scraped off coating solution is recovered and recirculated for reuse.
It is believed the reason such coating methods have been exclusively employed is because these methods enable thin layer coating at a high speed above 300 m/min. using a relatively simple operation and because a very high quality is not required for the products per se.
However, it is impossible to sufficiently achieve the function of a protective agent as mentioned above, when such coating methods are employed. That is, in the case of an air knife coating method, a protective agent of a large particle size is scraped off to a greater extent by a classification effect due to the air knife; also in the case of a blade coating method, a classification effect occurs at the spacing between the blade and the web, and a protective agent which has a high probability of contact with the blade is scraped off to a greater extent. It is extremely difficult to coat the protective agent in a desired amount, ultimately. In addition, this classification effect due to an air knife or a blade becomes more remarkable as the coating speed increases so that it is actually impossible to comply with recent demands for increases in production rates.
Furthermore, in general, the excess amount of the coating solution which is scraped off with an air knife or a blade is recovered and recirculated for reuse. However, the concentration of solid materials in the coating solution is gradually increased by the above-mentioned classification effect so that a coating composition changes with the passage of time. Therefore, it is extremely difficult to coat a coating solution with a constant composition on a web over a long period of time. It is thus impossible to prepare a pressure-sensitive copying paper of high quality.
In addition, the protective agent must be present near the surface of a coated layer, since the function of the protective agent is to protect the microcapsules from pressure or abrasion other than the localized pressure employed when such is used. However, the protective agent which is scraped off because of the above-mentioned classification effect is present near the surface of a coated layer. Accordingly, there is a problem that a sufficient protective effect is not obtained with a pressure-sensitive copying paper prepared by such methods.
In addition, there is a defect that copying capability as a pressure-sensitive copying paper is remarkably deteriorated, since all of these methods involve previously coating a coating solution on a web in a large amount and scraping the excess of the coating solution off and adjusting the amount to the coating amount desired and the coating solution permiates into the web to cause the web to swell.
Furthermore, in the case of an air knife coating method, the defects that not only is recovery difficult because the coating solution scraped off with an air knife is converted to a fine mist, e.g., very fine droplets, as the coating speed increases, and at the same time, the working environment is polluted or the edge of the air knife is contaminated causing streaks on the coated surface to occur, but also a limitation in the viscosity of the coating solution exists and a sufficient effect cannot be achieved in adjusting the amount of the coating solution having a high concentration and a high viscosity coated, and the like. In addition, in the case of a blade coating method, defects are also observed that coating for a long period of time in a stable manner becomes difficult due to abrasion of the blade and contamination of the blade.
Further, as mentioned above, it is desired for a pressure-sensitive copying paper not to be colored in handling and to color only when appropriately used. Conversely, efficient coloration upon appropriate use such as letter marking or the like must occur. For this purpose, the pressure-sensitive copying paper must have the characteristics that it is colored when a definite amount of pressure is applied but it is not colored when less than a certain amount of pressure is applied. Such a characteristic cannot be achieved with a pressure-sensitive copying paper prepared in accordance with conventional methods since the protective agent is selectively scraped off due to the classification effect as mentioned above.
Recently, a self-contained type pressure-sensitive copying paper has been employed which comprises a support having provided, on one side only, a coating layer containing microcapsules as a main component, as a lower layer, and a coating layer containing a color developer as a main component, as an upper layer. However, such a self-contained type pressure-sensitive copying paper is extremely complex and is uneconomical since it is prepared by a method which comprises coating a coating solution containing microcapsules as a main component on a web to form a coated layer, drying the coated layer and coating a coating solution containing a color developer as a main component on the above-mentioned coated layer to form a coated layer and then drying the coated layer, and thus two coating steps and two drying steps are involved.